Columbia University DECSYSTEM-20

From 1977 to 1988 the DECSYSTEM-20 dominated Columbia computing culture. It
brought us everything we take for granted today — interactive (rather
than batch) computing, file sharing, email, discussion groups, visual text
editing and formatting, spell checkers, production and typesetting of papers
and books, local and wide-area networking — everything except the
graphical user interface. Eventually we had four DEC-20s, plus another one
in the CS department and another at Teachers College, all networked
together, each costing about a million dollars.

The DEC-20 is a bit over 5 feet high and 10-30 feet wide depending on how
many cabinets for memory, tapes, networking, etc, plus any number of
washing-machine size disk drives, big line printers, and other peripherals (still, it was tiny compared to
the IBM 360/91 in the next room). It requires three-phase power and
isolated true-earth ground, raised floor, massive amounts of air
conditioning, and weekly or biweekly "preventive maintenance." It has two
million 36-bit words of main memory and accommodates 30 simultaneous users
comfortably, and up to about 100 with increasing discomfort.

As you can see, there is not much in the way of lights and switches. But
inside the main cabinet, not visible in the top picture, is the PDP-11/40
front end (used mainly to interface terminals and printers), which has a more
entertaining front panel done in shades of blue:

The switches labled 15 through 0 could be used to load 0's or 1's into the
16-bit switch register, which could then be deposited into PDP-11 (not PDP-10)
memory by using the the other switches (not that this was done commonly on the
DEC-20; earlier computers, however, were booted by "toggling in" the bootstrap
program in a like manner).

This shows the old VT52 terminal we kept on top of
CU20A to display the current load average (the higher the number, the slower
the computer; sometimes it got up past 100, where it would take several
minutes for each keystroke to echo). "Load average" means the average ratio of
working set to balance set over some period of time, like 1, 5, or 10 minutes.

Here's a full view of a small DEC-20 system from the vendor literature:

From left to right: An LP20F line printer, two TU45
tape drives, then the DEC-20 itself (front-end cabinet with serial line
interfaces; disk/tape interface and memory cabinet; KL10 CPU cabinet), a CD20
card reader (ours didn't have any of these), and two
RP04 disk drives. The terminals are a VT52 video terminal (DEC's first video terminal that had
both upper AND lowercase letters) and an LA36
DECwriter hardcopy terminal. Between them is an RP04 3330 disk pack (92MB).

Here is a floor plan drawn by Bill Schilit in 1984, when we had three DEC-20s
(explained HERE).